And Detective Munch from Law and Order: SVU (who was originally in Homicide: Life on the Streets) once appeared in an episode of The Lone Gunmen, a spinoff of The X-Files.

A fun little fact: As of his upcoming appearance on on Paris enquêtes criminelles (the French adaptation of L&O: Criminal Intent, he will hold the record with for the most appearances of a single actor as the same character on different TV show. He actually holds the record now at eight, but it will be an even bigger record (obviously) after this next appearce.

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They say a little knowledge is a dangerous thing, but it is not one half so bad as a lot of ignorance. - Terry Pratchett, Equal Rites